1. Field of the Invention
This invention relates to electrodes and a process for manufacture thereof, and more particularly to electrochemical air cathodes for use in electrochemical cells and a process for manufacture thereof.
2. Background Information
A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, natural gas, alcohol, or metal sheet, and an oxidant, such as oxygen or air. Since the process does not burn the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes.
Metal/air fuel cells (See, for example, U.S. Pat. No. 5,250,370 to S. Faris, which is fully incorporated herein by reference) and batteries produce electricity by electrochemically coupling in a cell a reactive metallic anode to an air cathode through a suitable electrolyte. As is well known in the art, an air cathode is typically a sheet-like member having opposite surfaces respectively exposed to the atmosphere and to an aqueous electrolyte of the cell, in which (during cell operation) atmospheric oxygen dissociates while metal of the anode oxidizes providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially hydrophobic (so that aqueous electrolyte will not seep or leak through it), and must incorporate an electrically conductive element for external circuitry. See, for example, U.S. Pat. No. 5,053,375 to Bhaskara M. L. Rao.
As described in U.S. Pat. No. 4,906,535 in present-day commercial metal-air electrochemical cell practice, the air cathode is commonly constituted of active carbon (with or without an added dissociation-promotion catalyst) containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals are used including iron, zinc, magnesium, aluminum, alloys of aluminum, etc.
U.S. Pat. No. 4,129,633 discloses a method for preparing an electrode using a dry powder spread onto a moving web (dry method). Disadvantageously, this approach generally requires the use of relatively complex equipment, and it tends to be difficult to distribute the dry powder uniformly.
U.S. Pat. No. 4,339,325 to Soloman et al. describes gas diffusion electrodes and methods for the preparation thereof. This reference describes and claims a porous, coherent, unsintered, uniaxially oriented backing layer of fibrillated polytetrafluoroethylene having openings ranging from about 0.1 to 40 microns. The layer may be used as a backing layer in forming an electrode. However, the backing layer generally does not provide the structure and strength desired for relatively demanding applications.
U.S. Pat. No. 4,615,954 discloses an oxygen cathode comprising: an electrically conductive, wetproofing layer composed essentially of an intimate, consolidated and heat sintered mixture of carbon black and particulate hydrophobic polymeric binder derived predominantly from tetrafluoroethylene, having at least one anisometric electroconductive reinforcing material incorporated therein. U.S. Pat. No. 4,877,694 discloses an electrode comprising a porous, gas supplying layer containing hydrophobic polymer, and an electrolyte porous active layer comprising catalyst containing carbon particles intimately blended with hydrophilic halogenated polymer binder for catalyzed carbon particles, which intimate blend is combined in said active layer with particulate substance bound with hydrophobic polymer. In U.S. Pat. No. 4,927,514 there is disclosed an electrode in multi-layer form and having enhanced inter-layer bonding, the electrode comprising a gas porous, polymer-containing support layer, a catalyst-containing and polymer-containing active layer and a gas porous intermediate bonding layer consisting of thermoplastic hydrophobic polymer. All three of these patents rely on a relatively complex, multi-stage process for preparing the electrodes, which process is characterized by forming a hydrophobic support layer that is dried and sintered, and then depositing a further active layer on the dry support layer which is also then dried and sintered.
U.S. Pat. No. 5,312,701 discloses a relatively complex batch-style or single pass fabrication process for preparing a gas diffusion electrode for metal-air batteries and fuel cells.
U.S. Pat. No. 4,885,217 discloses a continuous web-coating method for preparing an air cathode which is comprised of a sheetlike laminate including two carbon layers having opposed surfaces, respectively exposed for contact with a liquid electrolyte and with air, and optionally having a hydrophobic microporous film. This construction uses a carbon felt skeleton to which other components are added. Although this process tends to be cost-effective for some applications, the operating current densities of the air cathode typically only range from 50 mA/cm2 to 150 mA/cm2.
A need exists for an electrode for electrochemical cells, and process for fabricating the electrode, that provides improved structural characteristics, that may be fabricated using cost-effective continuous processing, and which exhibits operating current densities of over about 200 mA/cm2.
According to an embodiment of the present invention, an electrode for an electrochemical cell is provided. The electrode includes a current collector having a plurality of interconnected pores disposed therein, and a mixture of carbon particles and polymeric material located within the pores. The polymeric material is sintered in-situ within the pores.
In a variation of this aspect of the present invention, the electrode also includes a hydrophobic microporous membrane superposed with the current collector. In addition, the current collector is fabricated from metallic foam.
In another aspect of the present invention, a method is provided for forming an electrode for an electrochemical cell. The method includes the steps of:
providing a current collector having a plurality of interconnected pores;
disposing a carbon/polymer blend within the pores of the substrate; and
sintering the polymeric binder of the carbon/polymer blend in-situ with the pores of the current collector.
The above and other features and advantages of this invention will be more readily apparent from a reading of the following detailed description of various aspects of the invention taken in conjunction with the accompanying drawings.